Heretofore, organic piezoelectric material having piezoelectric and pyroelectric performances was well-known which can transduce thermal or mechanical stimulation into electric energy, and can be employed as audio equipment such as diaphragm for microphone or speaker, various heat sensor, pressure sensor, or instruments such as infrared detector, ultrasound probe, and vibration sensor having high sensitivity for detecting such as variation of gene or protein.
As piezoelectric and pyroelectric materials, employed are so called inorganic piezoelectric materials prepared by polarizing treatment of single crystal such as crystal, LiNbO3, KNbO3; thin film such as ZnO or AIN; and sintered body such as Pb(Zr, Ti)O3. These piezoelectric materials of inorganic composition have features of high elastic stiffness, high mechanical loss coefficient, high density and high dielectric constant.
On the other hand, also developed are organic piezoelectric materials such as polyvinylidene fluoride (hereinafter, referred to as “PVDF”) or polycyano vinylidene (hereinafter, referred to as “PVDCN”) (for example, Patent Document 1). As organic piezoelectric material has features such that workability for thin film or large area is excellent, arbitrary shape and form can be easily formed, and elasticity and dielectric constant is low, thereby it has feature to enable high sensitive detection when it is employed for sensor use.
Since organic piezoelectric material looses piezoelectric and pyroelectric property due to its low heat resistance as well as reducing physical property such as elastic stiffness at high temperature, it has limitations to applicable temperature range.
With respect to resolve these limitation, variously investigated were polyurea resin composition comprising urea bond as organic piezoelectric material, because it has large dipole moment at urea bond and exhibits excellent temperature characteristics as resin.
For example, disclosed was a so called deposition polymerization method in which polyurea film was formed by depositing isocyanate compound such as 4,4′-diphenylmethane diisocyanate (MDI) and diamine compound such as 4,4′-diamino diphenyl methane (MDA) simultaneously (for example, Patent Documents 2 and 3). However, since polyurea resin composition formed by the described deposition polymerization method has un-uniform molecular weight of formed olygomer or polymer, when polymerization was performed while polarizing treatment, polyurea resin composition was formed in a state of insufficient orientation. Therefore, it is difficult to utilize dipole moment of urea bond effectively, thereby further improvement was required to organic piezoelectric materials.
Dendrimer compound is a collective term of branched polymer having regular dendritic branches detailed in documents such as Hawker, C. J. et al: J. Chem. Soc., Chem. Commun., 1990, 1010; Tomalia, D. A. et al: Angew. Chem. Int. Ed. Engl., vol: 29, page: 138 (1990); Frech et. J. M. J.: Science, vol: 263, page: 1710 (1994), or Kakimoto Masaaki, Kagaku, vol: 50, page: 608 (1995). These molecules have polymer structure having regular branches from a center of molecule. Therefore, for example, as explained in above document by Tomalia, terminal of branches becomes extremely sterically-congested according to polymerization, resulting in forming spherically-extended molecular structure.
One of means for preparing piezoelectric material having high orientation is to introduce a polarizing group having high dipole moment into dendrimer compound to subject piezoelectricity. However, though there were reports of dendrimer compound having polarizing group (for example, Patent Document 4 and Non-Patent Document 1), examples related to piezoelectricity were not disclosed and there was no report related to dendrimer compound having enough performances as piezoelectric material. In view of the foregoing, the inventors of the present invention conducted investigations, and as a result, found that dendrimer can be employed as piezoelectric material.
However, when dendrimer was singly employed, it was found that there remained room for improvement in view of film forming and handling. Therefore, dendrimer would be employed by mixing with other base material having excellent film forming property. However, it was found that mixing with base material having low compatibility (large difference between ClogPs) caused phase separation between materials, resulting in low piezoelectricity and being difficult in applying to sensor described before, due to acoustic scattering caused at boundary.
Further, on the other hand, in an investigation of an organic piezoelectric material which has liquid crystal property by utilizing high dipole moment of urea bond, the inventor of the present invention found that organic material having liquid crystal property can be employable as piezoelectric material.
However, organic piezoelectric material having urea group had poor film forming property and also extremely bad handling. Therefore, organic piezoelectric material having urea group would be employed by mixing with other base material having excellent film forming property. However, it was found that mixing with base material having low compatibility (large difference between ClogPs) caused phase separation between materials, resulting in low piezoelectricity and being difficult in applying to sensor described before, due to acoustic scattering caused at boundary.